Go to your local Quick Print shop and have them print a 2 part carbonless business form that is filled out by your employee, keeping the original and giving the customer the 2nd copy.
The employee is required to turn in all receipts with the money at the end of each day.
You design the form and have all the needed information on the form, especially their phone number. You can follow-up several customers to ask how they were treated by your employees, were they satisfied with the job, etc.
If you wanted even tighter control, sequentially number each form and note the numbers
you give the employees and expect all the numbers that were given out in the morning will return in the evening. (Without this numbering, you will have no idea if they did a job and pocketed the money.)